A semiconductor thin film such as polycrystalline silicon (poly-Si) is widely used for a thin-film transistor (TFT) and a solar cell in related art. In particular, the poly-Si TFT is widely used as a switching device included in a pixel circuit for a liquid crystal display device, a liquid crystal projector, an organic EL display device and so on, or a circuit device for a liquid crystal driver by utilizing characteristics that the TFT has high carrier mobility and can be fabricated on a transparent insulating substrate such as a glass substrate.
As a method of fabricating the high-performance TFT on the glass substrate, there is a manufacturing method generally called a “high-temperature process”. In manufacturing processes of the TFT, a process performed at a high temperature of approximately 1000° C. as the maximum temperature in the operation is generally called the “high-temperature process”. Characteristics of the high-temperature process are a point that relatively-high quality polycrystalline silicon can be deposited by solid phase growth of silicon, a point that a high-quality gate insulating layer can be obtained by thermal oxidation of silicon and a point that an interface between clean polycrystalline silicon and the gate insulating layer can be formed. It is possible to stably manufacture the high-performance TFT with high mobility and reliability according to the characteristics of the high-temperature process.
On the other hand, as the high-temperature process is a process performing crystallization of a silicon film by the solid phase growth, thermal processing at a temperature of approximately 600° C. for a long period of time of approximately 48 hours is necessary. This is a process of the very long period of time, and many heat treatment furnaces are consequently necessary for increasing a throughput of the process, therefore, there is a problem that it is difficult to reduce costs. Additionally, as quartz glass must be used as the insulating substrate having high heat resistance, the substrate costs high and is not suitable for increasing the area of the substrate.
On the other hand, a technique for reducing the maximum temperature in the process and fabricating the poly-SiTFT on an inexpensive glass substrate having a large area is called a “low-temperature process”. In manufacturing processes of the TFT, a process of manufacturing the poly-SiTFT on the relatively inexpensive heat-resistance glass substrate under a temperature environment in which the maximum temperature is approximately 600° C. or less is generally called the “low-temperature process”. In the low-temperature process, a laser crystallization technique for performing crystallization of the silicon film by using a pulse laser having an extremely-short oscillation time is widely used. The laser crystallization is a technique of utilizing a nature that the silicon thin film on the substrate is melted at once by irradiating the film with a high-output pulse laser beam and the silicon thin film is crystallized in the process of being solidified.
However, there are some large problems in the laser crystallization technique. One problem is that many trapping levels locally existing inside a polysilicon film formed by the laser crystallization technique. Due to the existence of trapping levels, carriers which should normally be moved in an active layer by voltage application are trapped, which does not contribute to conduction of electricity, as a result, adverse effects such as the reduction of the mobility and the increase a threshold voltage of the TFT may occur. Furthermore, there is also a problem that the size of the glass substrate is limited due to the limitation of laser output. It is necessary to increase the area which can be crystallized at one time in order to improve the throughput of the laser crystallization process. However, in the case where the crystallization technique is applied to a large-sized substrate such as the seventh generation (1800 mm×2100 mm), a long period of time is necessary for crystallizing a piece of substrate as there is the limitation in laser output in the present condition.
The laser crystallization technique generally uses a laser formed in a line shape, and the crystallization is performed by performing scanning by the laser. The line beam is shorter than the width of the substrate as there is the limitation in laser output, and it is necessary to scan the substrate by the laser at plural times for crystallizing the entire surface of the substrate. Accordingly, a region of a joint between line beams is generated in the substrate, and there exists a region to be scanned twice. The region largely differs from the region crystallized by the scanning of one time in crystallinity. Therefore, device characteristics of both regions largely differ, which will be a major factor of device variations.
Lastly, there is a problem that the device costs and the running costs are high as the device configuration is complicated as well as the cost of consumable components is high in the laser crystallization device. Accordingly, the TFT using the polysilicon film crystallized by the laser crystallization device will be a device manufacturing costs of which are high.
In order to solve the problems that the substrate size is limited and that the device costs are high, a crystallization technique called a “thermal-plasma jet crystallization method” has been studied (for example, refer to Non-Patent Literature 1). The present technique will be briefly explained below. When a tungsten (W) cathode is arranged opposite to a copper (Cu) anode which is cooled by water and a DC voltage is applied, an arc discharge occurs between both electrodes. The thermal plasma is jetted from a jetting hole disposed on the copper anode by allowing an argon gas to flow between both electrodes under an atmospheric pressure.
The thermal plasma is thermal equilibrium plasma, which is a heat source of ultra-high temperature, in which temperatures of ions, electrons, neutral atoms are almost equivalent and these temperatures are approximately 10000 K. Accordingly, the thermal plasma can easily heat a heat-treated object to a high temperature, and a substrate on which an a-Si (amorphous silicon) film is deposited is scanned in front of high-temperature thermal plasma at high speed, thereby crystallizing the a-Si film.
As the device configuration is extremely simple and the crystallization process is performed under the atmospheric pressure as described above, it is not necessary to cover the device with an expensive member such as an airtight chamber, and it can be expected that the device costs will be drastically reduced. Additionally, utilities necessary for crystallization are the argon gas, electricity and cooling water, therefore, the running costs are low in the crystallization technique.
FIG. 19 is a schematic view for explaining a crystallization method of a semiconductor film using the thermal plasma.
In the drawing, a thermal plasma generation device 31 is configured by including a cathode 32 and an anode 33 arranged opposite to the cathode 32 so as to be apart from each other by a given distance. The cathode 32 is made of a conductor such as tungsten. The anode 33 is made of a conductor such as copper. The anode 33 is formed to have a hollow, which can be cooled by allowing water to flow into the hollow portion. The anode 33 is also provided with a jetting hole (nozzle) 34. When a DC voltage is applied between the cathode 32 and the anode 33, the arc discharge occurs between both electrodes. A gas such as an argon gas is allowed to flow between the cathode 32 and the anode 33 under the atmospheric pressure in the above state, thereby jetting a thermal plasma 35 from the jetting hole 34.
Here, the thermal plasma is thermal equilibrium plasma, which is a heat source of ultra-high temperature, in which temperatures of ions, electrons, neutral atoms are almost equivalent and these temperatures are approximately 10000 K.
The above thermal plasma can be used for heat treatment for crystallizing the semiconductor film. Specifically, a semiconductor film 37 (for example, an amorphous silicon film) is formed on a substrate 36, and the thermal plasma (thermal plasma jet) 35 is applied to the semiconductor film 37. At this time, the thermal plasma 35 is applied to the semiconductor film 37 while relatively moving the thermal plasma 35 along a first axis (right and left direction in the shown example) parallel to the surface of the semiconductor film 37. That is, the thermal plasma 35 is applied to the semiconductor film 37 while being scanned in the first axis direction.
Here, “to relatively move” means that the semiconductor film 37 (and the substrate 36 supporting the film) and the thermal plasma 35 are relatively moved, including a case where one of them is moved and a case where both of them are moved. The semiconductor film 37 is heated by high-temperature possessed by the thermal plasma 35 by the scanning of the thermal plasma 35, thereby obtaining the crystallized semiconductor film 38 (a polysilicon film in the example) (for example, refer to Patent Literature 1).
FIG. 20 is a conceptual graph showing the relation between the depth from the outermost surface of the substrate and the temperature. As shown in the drawing, only the vicinity of the surface can be treated at high temperature by moving the thermal plasma 35 at high speed. After the thermal plasma 35 passes by, the heated region is rapidly cooled, therefore, the vicinity of the surface becomes high temperature for a short period of time.
It is general that the above thermal plasma is generated at dotted regions. The thermal plasma is maintained by discharging thermal electrons from the cathode 32. Accordingly, thermal electrons are discharged more actively in a position where the plasma density is high, therefore, positive feedback is performed and the plasma density becomes higher. That is, the arc discharge occurs in a state of being concentrated at one point of the cathode, and the thermal plasma is generated at dotted regions.
Though it is necessary to scan the entire substrate by the dotted thermal plasma in the case where a flat-plate shaped substrate is uniformly processed such as crystallization of the semiconductor film, and it is effective that a region irradiated by the thermal plasma is increased for constructing the process which can perform processing for a short period of time by reducing the number of scanning times. Accordingly, a technique of generating a long thermal plasma and scanning only in one direction has been considered (for example, refer to Patent Literatures 2 to 7).